PreColumbian societies in the Maya Lowlands were repeatedly exposed to tephrafall from past eruptions of El Chichón volcano, Chiapas, Mexico. Late Holocene tephra layers are well preserved in sediments of the UsumacintaGrijalva river delta, about 150 km NNE of the volcano. A sediment core from the delta spans the last 2100 years, and was examined for tephra, pollen, palynomorphs and macrofossils.

We used tephrochronology and palynology to reconstruct the timing, magnitude and impact of past eruptions. Seven of the ten recognised tephra layers are related to past eruptions of El Chichón volcano and show remarkable impacts of the eruptions. A large eruption around A. D. 539 had a dramatic effect on the core area of Maya civilisation and was probably responsible for the onset of the Maya Hiatus and a demographic shift towards the UsumacintaGrijalva delta. The eruption also seems to have caused rapid environmental changes worldwide.

In 1982 El Chichón volcano had a violent plinian eruption that devastated an area of approximately 153 km2 surrounding the volcano (Sigurdsson et al., 1984). Besides the impact near the volcano, a large area was affected by tephra fall. Eastern Mexico was covered by fine white ash that produced total darkness in the adjacent state of Tabasco, close to our research site. The 5 mm isopach for the 1982 tephra fall, measured during and immediately after the eruption (Fig. 1), indicates an even larger affected area in comparison with earlier published data (Varekamp et al., 1984).

The plinian nature of the eruption and the extremely high sulphur content of the magma (Luhr et al., 1984) caused the emission of large quantities of sulphur into the stratosphere as detected by the Nimbus 7 satellite (Krueger, 1983). The stratospheric cloud circled the earth in three weeks (Robock and Matson, 1984) and subsequent deposition of aerosol particles caused sulphate peaks that were registered in snow pits and ice cores from Greenland (Zielinski et al., 1997), Antarctica (Traufetter et al., 2004) and the Himalayas (Hou Shugui et al., 2003).

Several authors demonstrated that El Chichón frequently erupted during the Holocene (Tilling et al., 1984; Macías, 1994; Espíndola et al., 2000). Espíndola et al (2000) identified 12 explosive eruptions of the volcano that occurred at approximately 550, 900, 1250, 1500, 1600, 1900, 2000, 2500, 3100, 3700 and 7700 years before present (BP). The ~550, ~1250 and ~3700 yr BP eruptions were larger or similar in magnitude than the 1982 eruption which had a VEI (Volcanic Explosive Index) between 4 and 5 (Macías et al., 2008). Only the ~550 and ~1250 yr BP eruptions have so far been linked to plinian activity that dispersed large amounts of tephra, as the 1982 eruption did (Espíndola et al., 2000; Macías et al., 2003). The impact of ancient eruptions on Maya societies is largely unknown and only a few attempts have been made to link past eruptions with the archaeological record (Gill and Keating, 2002).

In this paper we reconstruct the tephrochronological archive from the UsumacintaGrijalva delta. Tephrostratigraphy, 14Cdating and palynology were used to estimate the timing and magnitude of past volcanic eruptions. Microprobe analyses were used to identify the source volcano for one of the most distinct tephra layers. The impact of past eruptions was estimated and correlated with demographic changes within the Maya Lowlands.

We are not aware of any palaeoecological or palaeolimnological work in the Maya Lowlands that included the study of tephra. Although tephra may not always show up as distinct layers, a petrographic study by SteenMcIntyre (Sheets, 1983) demonstrated the presence of tephra within the sediments of lake YaxháSacnab, northern Guatemala (Fig. 1b). The widespread use of airfall tephra as ceramic temper throughout Maya history (Simmons and Brem, 1979; Ford and Rose, 1995) also indicates that tephra deposition was common in the Maya Lowlands.

The combination of tephrostratigraphy and AMS 14Cdating allows for the construction of a tephrochronological framework. Tephrochronology may help establish an accurate time chronology for the Maya Lowlands. This would be a boon for Maya archaeologists because conventional radiocarbon dating is less accurate, and for the Maya Lowlands even more problematic because of the "old carbon effect".

Materials and methods

In 2000, twelve cores were taken in the UsumacintaGrijalva delta, about 150 km NNE of El Chichón volcano. The cores were taken primarily to assess spatial and temporal variation in sediment accumulation rates. Many cores, however, contained distinct volcanic ash layers and it became clear that the delta sediments possess an important tephrostratigraphic archive. One site was of particular interest because of its distinct tephrostratigraphy. The site, Pozo Petrolero (18°29'72"N, 92°31'43"W) is located in a distal marshy flood basin between the Usumacinta and the San Pedro y San Pablo rivers (Fig. 1). Two 120cmlong parallel cores were taken with a 5cmdiameter corer in 66 cm of standing water. After transport to the Netherlands, the cores were stored in a dark room at 4°C. Loss on Ignition (LOI), an estimate of organic matter content, was determined by heating weighed samples at 550°C for four hours and reweighing (Heiri et al., 2001).

Pollen samples were taken every 1 to 5 cm and prepared using standard palynological extraction procedures, but without a HF treatment (Faegri and I versen, 1989). For each sampled depth a complete slide was counted for pollen and palynomorphs at 400x magnification. The diatoms that were present in the pollen slides were counted along with the pollen and other palynomorphs. Macrofossils were isolated from 2cm thick samples using a 5% KOH treatment and a 150µm sieve. Abundance classes for volcanic glass shards, charcoal and palynomorphs were estimated from the macrofossil samples and the pollen slides.

Glass shards from two tephra layers at 32 and 60 cm depth were isolated from untreated tephra samples by hand picking. Microprobe analyses of the glass shards were carried out at Laboratorio Universitario de Petrología, UNAM. Fortyfive microprobe glass analyses from twelve shards were made on the Jeol JXA 8900R with beams of 10 and 15 microns. The composition of the glass shards was compared with data from deposits of nearby volcanoes to identify the source volcano.

Six samples of terrestrial macrofossils and one bulk material sample were selected for AMS radiocarbon dating. The AMS 14Cdates were calibrated to calendar years with Oxcal 3.10 (Bronk Ramsey, 1995; 2000) using the IntCal04 calibration curve (Reimer et al., 2004).

Results and discussion

Both cores show the same lithology with fine beach ridge sands at their base (Fig. 2). As a result of relative sealevel rise, these ridges were gradually submerged and organic accumulation started about 2100 years ago. The results of the AMSradiocarbon dates (Table 1) and the agedepth model (Fig. 2) indicate that, except for the unconsolidated upper 20 cm, organic accumulation rates gradually decreased from 0.5 to 0.3 mm/year.

Three tephra layers, at 1, 32 and 60 cm depth, were very obvious and visible with the naked eye.

Other tephra layers were identified on the basis of peaks in the abundance of volcanic glass shards within the macrofossil samples and the pollen slides. We describe the tephra layers and the environmental changes associated with them as inferred from the analysis of pollen (Fig. 3), palynomorphs and macrofossils (Fig. 4).

The tephra layers are related to past eruptions of El Chichón (Table 2) or other volcanoes by comparing the timing of tephra deposition with the estimated eruption data from nearby volcanoes. One tephra layer could be related to a particular volcano according to the results of the microprobe analyses (Table 3 and Fig. 5). Possible relations with the archaeological data are given (Fig. 6). Finally, a tentative estimation of the eruption date is given by correlating the agedepth model with sulphate peaks in ice cores from Greenland (Zielinski et al., 1994; Clausen et al., 1997; Zielinski, 2000; Larsen et al., 2008) and Antarctica (Budner and ColeDai, 2003; ColeDai, 2004; Traufetter et al., 2004, Hofstede et al., 2004). Regarding the high sulphur content of older deposits (Rose et al., 1984), it is likely that large pre1982 eruptions also left a sulphate signal in ice cores.

El Chichón Tephra, unit H

Volcanic glass shards are abundant within the upper part of the sandy deposits (88>96 cm depth). Pollen is preserved poorly in sandy samples, but the taxa encountered indicate a turbulent sedimentation environment with early successional trees like Myrica cerifera and Mimosa pigra (zone 1). The sandy deposits contain abundant freshwater sponge megascleres and gemmoscleres from Racekiela sheilae indicating the presence of a shallow coastal wetland (VolkerRibeiro et al., 1988).

A wooded freshwater swamp (zone 2) followed the turbulent environment with tephra deposition (zone 1). The LOI gradually increases to ~25% and deposits are rich in wood fragments of Annona glabra. Pollen from Annona glabra is scarce, but present. The number of remains of the freshwater sponge Racekiela sheilae decreases and diatoms, characteristic of shallow freshwater habitats like Eunotia spp., Gomphonema spp. and Nitzschia spp., are abundant. The transition from zone 1 to 2 could be related partly to the eruption of El Chichón, but the transition is probably mainly caused by an increase in water level.

We relate the tephra to an eruption of El Chichón volcano (unit H). Although wet pyroclastic surges and ashflow deposits of unit H were found near the volcano, no fallout layers associated for this eruption have been identified previously (Espindola et al., 2000; Macías et al., 2008). The hydromagmatic eruption had an estimated VEI of 23 (Macías et al., 2008). The calibrated age of deposit Unit H is 30 ± 90 B.C. (Table 2) (Espíndola et al., 2000). A wood/charcoal sample from above the tephra layer yielded a calibrated age of 80 ± 90 B.C. (Table 1).

Although archaeological data for this period are relatively scarce, there are indications of a PreClassic stagnation and probably even a slight decrease in the formation of Maya polities between 100 and 0 B.C. (Fig. 6) (CioffiRevillia and Landman, 1999). Galop et al. (2003) found palaeoecological evidence for a decrease in agricultural activity around lake Tuspán (Fig. 1b) after approximately 100 B.C.

Within the time range of deposit H, the GISP 2 Greenland ice core shows three sulphate peaks. One of the largest sulphate peaks over the last 5000 yr. was recorded at 53 B.C. (Zielinski, 2000). The source volcano has not been identified yet, although a highlatitude Northern Hemisphere eruption is most likely given the absence of a sulphate peak in ice cores from Antarctica around this date (Hofstede et al., 2004). Two smaller sulphate peaks occurred in the GISP 2 core around 100 and 44 B.C. The 44 B.C. peak is related to an eruption of Etna volcano in Italy, which caused a dry fog in Italy and northern China (Stothers and Rampino, 1983; Stothers, 2002). We relate the 100 B.C. peak to an eruption of El Chichón. Around the same date, 114 B.C., Hofstede et al. (2004) found a small dielectric profiling (DEP)peak (indicative of sulphate) in an ice core from Antarctica.

El Chichón Tephra, unit G

A small peak in the abundance of volcanic glass shards within the macrofossil and pollen samples occurred at ~77 cm depth. The tephra layer marks a change from a temporarily flooded wetland environment to permanently flooded conditions (zone 3). Many shallow open water indicators like zoospores from charophytes, resting eggs of rotifers and copepods, diatoms and sponge megascleres and gemmoscleres disappear. Only spores of the filamentous green algae Mougeotia and Spirogyra remain. Cephalanthus occidentalis trees gradually replace Annona glabra trees, the former having a higher flood tolerance. Pollen from Cephalanthus occidentalis increases to ~60% at the end of zone 3.

The tephra layer at 77 cm depth is probably related to an eruption of El Chichón volcano and to wet pyroclastic surges and ashflow deposits of unit G found near the volcano (Espíndola et al., 2000). The hydromagmatic eruption had an estimated VEI of 23 and fallout layers have not been identified yet (Macias et al., 2008). The calibrated age of Unit G is A.D. 180 ± 60 (Table 2). We obtained a calibrated age of A.D. 195 ± 65 for a seeds/ charcoal sample from above the tephra layer.

The Preclassic Abandonment of important Maya cities like El Mirador (Guatemala),which was rapidly depopulated around A.D. 150 (Hansen et al., 2002), could be related to the eruption of El Chichón. Palynological evidence for this abandonment comes from a study of Lake Puerto Arturo, located nearby El Mirador. A minimum in disturbance indicators (Poaceae and Asteraceae) occurred at approximately A.D. 140 (Wahl et al., 2006). Other smaller centres like Cerros and Colha were also abandoned and Tikal and Lamanai experienced a period of stasis in construction (Freidel and Schele, 1988 and references therein).

Within the time range of deposit G there is not a clear sulphate signal in ice cores from Greenland and Antarctica that could be related to the El Chichón eruption. We tentatively assign the small signal around A.D. 159161 in the GISP2 and GRIP ice cores (Clausen et al., 1997; Zielinski, 2000) to the eruption of El Chichón. No signal was found in Antarctic ice cores around this date (Hofstede et al., 2004.

El Chichón Tephra, unit F

During the first analysis of the macrofossil samples no tephra was recognised at 68 cm depth. However, a small peak in the number of tephra shards in the pollen slides, a slight decrease in LOI, and the appearance of diatoms around 68 cm depth (Fig. 2) do point to a deposition of tephra at this depth. A macrofossil sample from the parallel core did confirm the presence of volcanic glass shards.

Although no major changes occurred in the pollen assemblage at this depth (Fig. 3) we identified a zone change (zone 3 to 4) on the basis of changes within the palynomorphs and macrofossil samples. Charcoal particles become abundant and the cyanobacterium Rivularia sp. and Arniumtype fungi ascospores show a marked increase, probably related to intensification of human activity in the area.

The tephra can be related to an eruption of El Chichón volcano and ash flow deposits of unit F (Espíndola et al., 2000). The eruption was probably hydromagmatic with a VEI of approximately 23 (Macías et al., 2008). Like the previous two eruptions no fallout layers associated with the eruption have been found yet. The calibrated age of deposit Unit F is A.D. 335 ± 85 (Espíndola et al., 2000).

Although the delta was already inhabited for many centuries, the eruption could have caused a significant migration towards the Usumacinta river and the delta. A stagnation in the erection of monuments occurred within the Maya Lowlands (Fig. 6) (Lowe, 1985) and archaeologists describe a period of political instability related to the arrival of the stranger 'Fire Is Born' in A.D. 378.

A 1.3cmthick yellowish tephra layer is present at 60 cm depth. The tephra contains rhyolithic sharpedged or vesiculated glass shards and angular crystals up to 400 pim in size.

The tephra layer was deposited within zone 4. The pollen diagram shows an enormous change in the pollen assemblage. High abundance of charcoal and the occurrence of a few pollen grains from Cucurbita sp. and Ipomoea sp. do indicate an increase of human presence in the area. Severe humaninduced burning caused the disappearance of trees like Cephalanthus occidentalis and temporary dominance of marshy vegetation by the fern Blechnum serrulatum. The pollen diagram shows an increase in fern spores of more than 80%. A time lag between the deposition of the tephra and the major vegetation changes indicates that the effect of the El Chichón eruption was indirect. There was probably a large demographic shift from inland locations toward the UsumacintaGrijalva delta. Archaeological data support this hypothesis. Gunn and Folan (2000) describe a large population shift during the Late Classic period (A. D. 600  800) when the Maya populations moved from inland locations toward the plains of the lower and middle Usumacinta River (including the San Pedro y San Pablo and Palizado rivers) and the coast. During the Terminal Classic period (A.D. 800  1100) these populations had their maximum influence along these rivers and the coast.

The glass shard chemical composition (Table 3) indicates that the glass shards are slightly more silicic and contain less Aluminium than the glass shards reported for El Chichón unit A (Cocheme et al., 1982; Luhr et al., 1984) and unit B (Macías et al., 2003). However they also show high potassium concentrations typical of El Chichón glass shards (Fig. 5), making El Chichón the most likely source volcano. The tephra layer is probably related to unit E, although no fallout layers have been associated with the massive block and ash flow deposits of unit E (Espíndola et al., 2000; Macías et al., 2008). The eruption has been assigned a VEI of 3 and probably included a dome destruction event (Macías et al., 2008). The average of four radiocarbondated samples (Table 2) yielded a calibrated age of A.D. 567 ± 37 (at 68.2 % probability). Probably the best sample was from carbonised tree branches from a pyroclastic flow deposit with white pumice which gave an AMS radiocarbon age of 1520 ± 75 14C yr. B.P. (A.D. 520 ± 90 at 68.2 % probability) (Espíndola et al., 2000). We eliminated the AMS radiocarbon age of 346 ± 34 14C yr. B.P. on a wood sample, probably a root, from directly above the ashlayer. A charcoal sample from the same depth gave an AMS radiocarbon age of 1535 ± 44 14C yr. B.P. (A.D. 460 ± 30 and A.D. 540 ± 40 at 68.2 % probability). This age is in line with the calibrated age for volcanic deposit unit E.

There was a worldwide reduction in tree growth between about A.D. 539 and A.D. 545 that was detected using treering analysis (Eronen et al., 2002; D'Arrigo, 2001; Baillie, 1994; Scuderi, 1990, 1993) and crop failure, famine and disease were widespread in Eurasia and Africa (Gunn, 2000).

This might have been the result of the sulphate aerosol cloud that was formed after the eruption. The stratospheric cloud probably spread globally and caused a strong reduction in direct solar irradiation worldwide.

It is remarkable that around the same time, a dense and persistent dry fog occurred in Europe and the Middle East. Historical records describe meteorological phenomena like those observed in A.D. 536537: "The sun was dark and its darkness lasted for eighteen months; each day it shone for about four hours; and still this light was only a feeble shadow... the fruits did not ripen and the wine tasted like sour grapes." (Stothers, 1984). However, it is not very likely that the stratospheric cloud of the El Chichón eruption caused this dry fog. Large plinian eruptions can cause climatic forcing, but most dry fogs are caused by minor events that inject toxic volatile gases and aerosols only into the troposphere (Grattan and Pyatt, 1999).

According to Maya archaeologists, the period after approximately A.D. 539 is known as the "Maya Hiatus." The "Maya Hiatus" was first noted by Morley (1938) as a period of diminished dedication of historical carved monuments (stelae) at ancient Maya sites (Fig. 6). During the "Hiatus," important cities like Tikal and cities along the Río de la Pasión and the Usumacinta River were devastated (Gill and Keating, 2002). For the Rio Azul area, the "Hiatus" was comparable to the Classic Maya Collapse of the 9th century AD, the only difference being that there was a recovery from the "Hiatus" (Adams, 2003). Adams (1990) showed that both elite centres and rural areas were devastated during the "Hiatus". Other evidence comes from a palynological study of a core from Lake Tuspán, a lake 300 km east of El Chichón volcano. Results of the pollen analyses show a drastic interruption of agricultural activity at approximately A.D. 540 (Galop et al., 2005), just after a high magnetic susceptibility peak.

A large volcanic sulphate peak was found in ice cores from Greenland around A.D. 529530 (Zielinski, 2000; Larsen et al., 2008). The peak was caused by a mid to high northern latitude eruption (Larsen et al., 2008) and was likely related to the dry fog of A.D. 536537. Larsen et al. (2008) also found a large volcanic sulphate peak in the Dye3, GRIP and NGRIP ice cores from Greenland at A.D. 533534 ± 2, using the new GICC05 timescale (Vinther et al., 2006). The peak was not found in the GISP2 ice core and previously underestimated in the GRIP core (Clausen et al., 1997). Probably the same volcanic eruption caused a large signal around A.D. 547 ± 17 in Antarctic ice cores (Traufetter et al., 2004). Given the amount and distribution of sulphate deposition, the eruption must have been a tropical volcanic eruption of somewhat larger magnitude than the A.D. 1815 Tambora eruption (Larsen et al., 2008). El Chichón was the probable source volcano and we tentatively assign A.D. 539 as the most likely year of eruption (Nooren et al., in prep.).

El Chichón Tephra, unit D

Although the number of volcanic glass shards in the pollen slides show a small peak at 56 cm depth, we did not find clear evidence of tephra deposition related to this large plinian eruption of ~1250 yr BP. The eruption left thick ash flow and surge deposits (unit D) near the volcano (Espíndola et al., 2000) and contained abundant pottery shards (Tilling et al., 1984), and had an estimated VEI of 4 (Macías et al., 2008). The calibrated age of deposit Unit D is A.D. 755 ± 75 and A.D. 855 ± 15 at 68.2% probability (Table 2) (Espíndola et al., 2000).

El Chichón Tephra, unit C

At a depth of about 45 cm, volcanic glass shards are abundant in the macrofossil samples. Changes in the pollen assemblage are comparable with the situation after the ~A.D. 539 eruption, although changes are less pronounced. Fern spores increase at the expense of Cephalanthus pollen. After the temporary decrease in Cephalanthus pollen, they rapidly regain their dominance and reach their highest percentages in the pollen diagram. The tephra marks the transition from zone 4 to zone 5. Relatively high abundance of Tetraedron minimum and an increase of Characeae oospores (Chara sp. and Nitella sp.) indicate a change to wetter conditions.

Although no clear sulphate peaks occurred in the GISP2 ice core (Zielinski, 2000), ice cores from Antarctica show peaks at A.D. 1040 ± 8 (Traufetter et al., 2004) and around A.D. 1045 (ColeDai, 2004).An eruption around A.D. 1175 would also be in accordance with the calibrated age of deposit unit C. Both ice cores from Greenland and Antarctica show large sulphate peaks around A.D. 1175. However this date does not fit well in our agedepth model, making an eruption around A.D. 1045 most likely (Fig. 2).

El Chichón Tephra, unit B

A small peak in the abundance of volcanic glass shards within the macrofossil samples occurred at ~37 cm depth. This tephra marks the change from zone 5 to 6. Cephalanthus pollen percentages decrease from more than 80% to less than 20% and they are replaced by a variety of pollen from Bucida buceras, Rhizophora mangle, Mimosa pigra, Typha domingensis and others. This might indicate a disturbance by increased salinity or drought. The absence of a fern peak makes this event different in comparison to the earlier human impact events. The exact environmental meaning of the decrease in Cephalanthus pollen at this level remains, however, unclear.

The tephra was probably deposited after the large plinian eruption of El Chichón around ~A.D. 1343 (unit B). The eruption (VEI ~4) mainly resulted in thick pumice fall deposits near the volcano (Macías et al., 2008). The calibrated age of deposit Unit B at 68.2% probability is A.D. 1335 ± 20 and A.D. 1395 ± 10 (Espíndola et al., 2000). The ~A.D. 1343 eruption column (~31 km) reached higher into the stratosphere than the three sustained plinian columns of the 1982 eruption (Macías et al., 2003). The distribution of the fallout layers indicates a NE orientation of the plume for which Palenque (see Fig. 1) must have been covered by at least 2 cm of ash (Macías et al., 2003). At that time, Palenque had already been abandoned for centuries.

Ice cores from Greenland and Antarctica show large sulphate peaks at A.D. 1345 ± 2 (Zielinski, 2000), A.D. 1343 ± 5 (Traufetter et al., 2004) and around A.D. 1347 (ColeDai, 2004), which coincides with the age of this tephra layer based on the agedepth model (Figure 2).

Unknown source volcano Tephra

A 2.4 cm thick, yellowish tephra layer is present from 30.532.9 cm depth within zone 6. The tephra layer contains sharpedged vesiculated glass shards and angular crystals of comparable size to those from the tephra layer at 60 cm depth. The tephra contains relatively more basaltic hornblende and augite.

The same tephra layer was found at many of the 12 coring sites, pointing to widespread deposition of ash. From the available AMS radiocarbon dates (Tebbens, unpublished data) we calibrated an approximate age of A. D. 1515 for the deposition of the tephra layer.

Remarkably, the eruption did not cause a major change in the pollen assemblage. The only noted change is a shortterm increase of freshwater sponges and diatoms, in response to the increased supply of silica after the tephra deposition. Likewise, Telford et al. (2004) found increased diatom and sponge concentrations after tephra deposition in central Mexican lake sediments.

Preliminary analyses of the glass shards indicate a different composition than those of the ~A.D. 539 eruption. The glass is more andesitic, has less silica and potassium, and more aluminium. Unfortunately we cannot identify the source volcano yet. Espíndola et al. (2000) did not find evidence of an eruption of El Chichón around this date.

A small peak in the abundance of tephra within the macrofossil samples was found at the transition from zone 6 to 7 at 22 cm. Bucida buceras pollen increases rapidly and dominates the tree pollen assemblage. In the herb pollen assemblage, only Cyperaceae, Poaceae and Polygalaceae show a slight increase. The disappearance of algal remains, poor preservation of pollen, and increase in fungal spores indicate a change to drier conditions.

The abrupt change probably marks the onset of a severe drought period in Mesoamerica that occurred after the 1793 San Martín and 1815 Tambora eruptions (Gill and Keating, 2002). Caballero et al. (2006) found shallower lake levels at Lago Verde between circa A. D. 1785 and 1885, indicating drier conditions in the area near San Martín volcano.

We tentatively adopt an age for this tephra layer of A. D. 1793, mainly based on the agedepth model (Figure 2). Nonetheless, the 1793 eruption of San Martín was probably local and phreatomagmatic (Espíndola, personal communication), which might have produced only a minor impact at the study site.

Santa María Tephra (A.D. 1902)

The abundance of tephra shards within the macrofossils in the pollen slides shows a peak at 15 cm depth. The peak is probably related to an eruption of Santa María in Guatemala in A. D. 1902. Historical records do describe tephra deposition in Tabasco after the A. D. 1902 eruption of Santa María, and the Pozo Petrolero site is located within the 1 mm tephra fall isopach (Williams and Self, 1983).

The eruption marks the change from zone 7 to 8. The return of Cephalanthus does indicate a change towards wetter conditions. However samples above 15 cm contain few pollen and are dominated by fungal spores probably related to an increase of human activity in the area. During the mid 19th century canals were dug and dikes were built to make the area more accessible.

Tephra El Chichón, unit A (A.D. 1982)

At approximately 1 cm depth a ~1 mm thick tephra layer was encountered. The tephra layer contains sharpedged vesiculate glass shards and angular crystals (including basaltic hornblende and augite) of comparable size as those from the tephra layer at 30 and 60 cm depth. Based on the shallow occurrence and the location of the coring site within the known distribution of the 1982 El Chichón 1mm isopach (Figure 1), it is highly likely that this tephra layer was deposited during the 1982 El Chichón eruption.

Conclusions

The Pozo Petrolero study site enabled us to build a tephrochronological framework which can be linked to the late Holocene eruption history of El Chichón volcano. Seven of the ten tephra layers could be related to past eruptions of El Chichón and correspond very well with units A, B, C, E, F, G and H as described by Espíndola et al. (2000). Although for many of these eruptions no fallout layers have been identified yet, our results indicate that most of the eruptions caused tephra fall at our study site, 150 km NNE of the volcano. Remarkably no clear evidence of tephra deposition was found at the study site after the large plinian eruption of ~1250 yr BP (unit D).

A tephra layer from an unknown volcano was deposited at approximately A. D. 1515.

Most eruptions could be related to sulphate peaks in ice cores from Greenland and Antarctica. However, further tephrochronological work in the area is needed to verify the tentative eruption data given.

Almost all tephra layers correlated with stratigraphic changes in the composition of the sediments. At this distant location, changes are not due to the direct impact of tephra fall, except for the shortterm increase of freshwater sponges and diatoms in response to an increased supply of silica. Changes are mainly caused by human responses in severely affected areas. The eruption around A.D. 539 for example, registered in the core at 60 cm depth, had a devastating effect on the core area of Maya civilisation and was probably the onset of the Maya Hiatus that was characterised by a demographic shift towards the UsumacintaGrijalva delta. The eruption also caused rapid environmental changes worldwide.

Limnological and palynological work at higher resolution, in combination with tephrochronology, will contribute to a more detailed understanding of the impact of past eruptions. We hope that this research will stimulate archaeologists to look for evidence of past eruptions of El Chichón. Eruptions that frequent and devastating must have been recorded by the ancient Maya.

Acknowledgements

Mark Brenner, José Luis Macías and Margarita Caballero are thanked for their valuable comments to improve the manuscript. We thank Klaas van der Borg of the Department of Subatomic Physics of the Utrecht University for carrying out the AMS radiocarbon dating. Henry Hooghiemstra, Timme Donders, Bas van Geel, Rob van Soest and Jifke KoekNoorman are thanked for their help with the identification of pollen, palynomorphs and macrofossils. Margarita Reyes and Carlos Linares helped with the probe analyses and Amiel Nieto with the drafting. This project was partly sponsored by a research grant from the Dutch Organisation for Scientific Research (NWOWOTRO) in the Netherlands.

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